1. Field of the Invention
The present invention relates to a flow rate-measuring apparatus for detecting the flow rate of a liquid on the basis of a difference in propagation velocity obtained when sonic waves are propagated via the liquid.
2. Description of the Related Art
A flow rate-measuring apparatus has been hitherto known, which comprises, for example, a pair of transmitter/receiver units arranged on upstream and downstream sides of a conduit through which a fluid flows, wherein ultrasonic waves transmitted from one transmitter/receiver unit are reflected by an inner wall surface of the conduit and the ultrasonic waves are received by the other transmitter/receiver unit in order to measure a flow velocity or flow rate of the fluid, on the basis of a difference in propagation velocity of the ultrasonic waves.
As shown in FIG. 7, such a flow rate-measuring apparatus 1 comprises a measuring conduit 4, which includes a supply conduit 2 for supplying the liquid and a discharge conduit 3 for discharging the liquid. A first measuring head 5 is provided at one end of the measuring conduit 4, and a second measuring head 6 is provided at the other end of the measuring conduit 4. The first and second measuring heads 5, 6 have transducers therein, which function as a sonic wave transmitter or a sonic wave receiver. For example, a pulsed sonic wave signal is transmitted from the first measuring head 5, wherein the second measuring head 6, which serves as the sonic wave receiver, receives the sonic wave signal. Subsequently, the first measuring head 5 is switched so as to function as a receiver, to receive a sonic wave signal transmitted from the second measuring head 6 (see Japanese Laid-Open Patent Publication No. 7-83715).
In the case of the conventional technique disclosed in Japanese Laid-Open Patent Publication No. 7-83715, when bubbles are contained in the liquid, such bubbles adhere to the inner wall surface of the measuring conduit 4, which is disposed in opposition to the first and second measuring heads 5, 6. The bubbles 7 inhibit propagation of the sonic wave signals, making it impossible for the first and second measuring heads 5, 6 to accurately receive the sonic wave signals. As a result, it is difficult to correctly measure the flow rate of the liquid on the basis of sonic wave signals.